1. Field of the Invention
The present invention relates to a thin 3-piece oil ring for internal combustion engines, having a corrugated spacer expander, and upper and lower side rails holding the spacer expander therebetween.
2. Description of the Prior Art
Generally, a 3-piece oil ring for internal combustion engines has a corrugated spacer expander and upper and lower side rails, where the corrugated spacer expander is of a circumferentially successive, substantially U-shaped configuration, including circumferentially spaced outer crowns and inner crowns, with each of the outer crowns connected with next adjacent ones of the inner crowns on opposite sides thereof by a pair of connecting members. The spacer expander has protrusions beyond the axial width of the outer crowns forming upper and lower, outer supporting members bearing against the lower and upper sides of the upper and lower side rails, at the outer portion thereof, respectively, and has extensions beyond the axial width of the inner crowns forming upper and lower, inner supporting members bearing against the inner edges of the upper and lower side rails, respectively. The inner supporting members are bent at a slight angle to the vertical inwardly and upwardly for the upper inner supporting members and inwardly and downwardly for the lower inner supporting members. The end portions of the spacer expander come together and abut each other, referred to as abutting members, enabling compression of the spacer expander when properly installed. Such spacer expander is formed into a generally circular form, initially greater in circumference than it will be when it has been circumferentially compressed and shortened on installation in an engine cylinder.
When such 3-piece oil ring is installed in a piston ring groove of a piston, and the piston with the ring therein within a cylinder of an internal combustion engine, the expansive force of the spacer expander provides a predetermined tension to the upper and lower side rails through the action of inclined inner supporting members If one of the abutting members of the spacer expander overlaps the other abutting member, however, the tension cannot be provided to the upper and lower side rails. To prevent this from occurring, an arrangement is disclosed in Japanese Patent Publication No. 1-52576 (1989), where a guide bar is circumferentially threaded through a plurality of the U-shapes of the spacer expander on both of the opposite sides of the abutment, and the guide bar is fixed to one or more of the U-and-inverted-U shapes.
To enable a thin 3-piece oil ring of a width not greater than 2 mm to exhibit a good oil-sealing characteristics, the radial and axial bearing pressures of the upper and lower side rails are respectively required to be circumferentially uniform. In conventional corrugated spacer expanders, however, it was found that the axial bearing pressure of the side rails generated by the spacer expander in the portion on and around one abutting member was extraordinarily greater than in other portions, and the axial bearing pressure of the side rails in the portion on and around the other abutting member of the spacer expander was extraordinarily smaller than in other portions.
A thorough research on possible causes for this by the present inventors, has revealed that the conventional corrugated spacer expander is apt to cause both of the abutting members at the abutment to be axially displaced and be twisted with each other, which in turn causes the bearing pressure at the inclined supporting members against the upper and lower side rails to be made greater with one side of the abutment and smaller with the other side of the abutment, thereby causing a great discrepancy in the axial bearing pressure between both of the opposite sides of the abutment.
The present invention is made taking the above into consideration, and an object of the present invention is to provide a thin 3-piece oil ring having a corrugated spacer expander of reduced discrepancy in the axial bearing pressure between both of the opposite sides of the abutment.